1. Field of the Invention
The present invention relates to a slit-scanning and image-projecting device to slit-scan information on a flat scanning plane by the use of a deflector, and to project the same on the surface of a light receiving member.
2. Description of the Prior Art
It has been well known that an optical device of a type, which scans a flat scanning plane in the form of a slit by use of a deflecting device such as rotatory or oscillatory reflecting mirror, and sequentially projects informations on the scanning surface to a light receiving surface, is capable of transmitting the information on the scanning surface to the light receiving surface at a high speed, and is therefore considered a very effective projecting device from the standpoints of vibration and mechanical construction.
FIG. 1 of the accompanying drawing is a schematic diagram showing one embodiment of a conventional scanning and projecting device using a rotatory mirror as the deflector, wherein the flat scanning surface 1 is scanned by the rotatory reflectng mirror 2 in the form of a slit-shaped scanning surface 3 with a breadth l.sub.O to be determined by a slit (not shown), and the slit-shaped scanning surface 3 is focussed on the a light receiving surface 6 by the projecting optical system 5. In case, however, the flat scanning surface is scanned by using the rotatory scanning mirror 2, there exists such a difficulty that the light path length between the scanning surface 1 and the light receiving surface 6 varies with rotation of the rotatory scanning mirror 2. In more detail, if it is assumed that the position of the rotatory scanning mirror when the distance between the rotatory scanning mirror 2 and the scanning surface 1 is the shortest (L.sub.O) be a reference position, and a distance between the scanning mirror 2 and the scanning surface 1 when the scanning mirror rotates by .theta. degree from the reference position be L.sub.O +.DELTA.L, the following relationship is established. ##EQU1## (where: .DELTA.L is a quantity of displacement in the light path length to occur by rotation of the scanning mirror 2, which reaches its maximum value of 41.4 mm in the case of, for example, .vertline..theta..vertline..ltoreq.25.degree., L.sub.o =400 which data can be contemplated usually.
As an expedient for correcting such variations in the light path length, there is disclosed in Japanese patent publication No. 50-9182 a device, in which parallelly moving mirrors are provided in the front and back positions of the projecting optical system covering a span from the rotatory reflecting mirror to an image plane, thereby correcting variations in the light path length, and moving the projecting optical system to eliminate variations in the conjugative layout.
Also, in U.S. Pat. No. 3,537,373, there is taught a device, in which two lens elements in the projecting optical system are synchronously actuated in parallel with the optical axis to remove variations in the light path length and the conjugative lay-out.
In case, however, of using the rotatory reflecting mirror in a reproduction apparatus, there is an additional difficulty besides the abovementioned variations in the light path length, i.e., tilting of the scanning plane 1 in accordance with a rotational angle of the rotatory reflecting mirror 2 as viewed from the side of the light receiving surface 6. In other words, considering a mirror image 4 of the slit-shaped scanning plane 3 scanned by the rotatory reflecting mirror 2, this mirror image 4 is disposed with inclination by the rotational angle .theta. of the rotatory reflecting mirror 2 relative to the optical axis of the projecting optical system 5. This state will hereinafter be called "tilting". Also, an image 7 on the light receiving surface 6 of this slit-shaped scanning plane 3 scanned by the projecting optical system 5 is tilted by the same angle .theta.. This phenomenon of blurred width is practically inacceptable, because, when the slit width l.sub.O is 10 mm, and the effective F-No. of the projecting optical system is 10, the blurred width .DELTA.l' of the slit image 7 on the photosensitive body 6 will amount to approximately 0.2 mm when the rotational angle .theta. is 25 degrees, as shown in FIG. 2.